In the field of mobile communication, next-generation communication systems as successors to W-CDMA and HSDPA are being discussed by 3GPP, a standardization group. Examples of next-generation communication systems include a Long Term Evolution (LTE) system and an IMT-advanced system (fourth-generation mobile communication system). The IMT-advanced system (IMT-A) is also called the LTE advanced system (LTE-A).
In response to demand for high-speed communications, system bandwidths have been increased. For example, for downlink, a system bandwidth of 5 MHz is used in W-CDMA and a maximum system bandwidth of 20 MHz is used in LTE. In a successor system to LTE, it may be necessary to further increase the communication speed and the system bandwidth. For example, compared with LTE where a transmission rate of about 300 Mbps (downlink) is achieved using the bandwidth of 20 MHz, a much wider bandwidth of 100 MHz may be necessary to achieve a transmission rate of 1-2 Gbps. Also, such a wide frequency band is necessary for each telecommunications operator.
Meanwhile, to achieve continuous two-way communication, it is preferable to employ a frequency division duplex (FDD) scheme where separate frequency bands are used for uplink and downlink. In this case, a bandwidth corresponding to about 3% of the center frequency is used as a separating band (center gap) to sufficiently suppress interference between an uplink signal and a downlink signal. Since only high-frequency bands are available for future mobile communication systems, it is necessary to provide a wider center gap to employ an FDD scheme. Assuming that the center frequency is 3.6 GHz, a bandwidth as large as about 130 MHz is used for the center gap.
Also, the propagation loss (path loss) of a signal increases as the frequency becomes higher. For example, attenuation of a signal in a 3 GHz band is greater than that of a signal in a 2 GHz band by about 6 dB. Therefore, in a system using a high frequency band, the cell radius (or cell coverage) tends to become small. One approach to improve the signal quality is to increase the transmission power. However, it is not practical to increase the transmission power to such a level that the above problem can be solved. Particularly, compared with a base station, it is difficult to increase the transmission power of a user device.
Roughly two types of channels are used in a mobile communication system: data channels and control channels. It is possible to improve the signal quality of a data channel to some extent by appropriately performing adaptive modulation and channel coding (AMC). However, unlike a data channel, it is difficult to effectively perform AMC to control the transmission rate of a control channel. Therefore, when considering the cell coverage, it is necessary to give more attention to control channels rather than to data channels.